Structural reinforcement for electronic substrate

ABSTRACT

Embodiments of stiffening members in accordance with the present invention provide a mechanical support that is wave soldered to the frontside of the system substrate simultaneously with other wave soldered components. The stiffening member comprises a flat plate with a plurality of mounting pins. The number of mounting pins are predetermined to provide the plate with sufficient support to resist expected loading conditions when wave soldered in plated through holes on a system substrate. The mounting pins are adapted for insertion into plated through holes on the system substrate. The length of the mounting pins are predetermined to account for the height of the SMT component upon which it is attached, the thickness of the system substrate, and the desired amount of mounting pin protrusion from the backside of the system substrate. The stiffening members consume very little system substrate space while retaining a platform for heat dissipation.

FIELD OF THE INVENTION

[0001] The present invention relates to electronic packaging and, moreparticularly, to structural reinforcement for electronic substrate.

BACKGROUND OF INVENTION

[0002] A microelectronic package comprises a microelectronic dieelectrically interconnected with a carrier substrate, and one or moreother elements, such as electrical interconnects, a die lid, a heatdissipation device, among others. An example of a microelectronicpackage is an integrated circuit microprocessor. Interconnectingcontacts on the carrier substrate with those on a system substrateprovides electrical communication between the microelectronic die andexternal components. An example of a system substrate is a printedcircuit board (PCB) in the form of a motherboard for a desktop computer.The system substrate is commonly assembled into a system that is housedwithin a frame or chassis, for example, the enclosure of a desktopcomputer.

[0003] More powerful microelectronic die that operate at increasinglyhigh temperatures require the use of larger and heavier heat dissipationdevices on the microelectronic package. An example of a heat dissipationdevice includes a heat sink. The added weight of the heat dissipationdevice can cause the system substrate to flex and sag, especially underdynamic loading conditions experienced during handling of the system.

[0004] Surface mount technology (SMT) interconnects are especiallysusceptible to damage due to system substrate flexing and sag. Oneexample of a SMT interconnect is the ball grid array (BGA) whereincontact pads on the surface of the SMT component are interconnected withcontact pads on the surface of the system substrate using solder balls.Any loading that would tend to cause one of the surfaces to becomenon-flat will put stress on the solder interconnections. For example, ona commonly used desktop motherboard, the BGA to board interconnects ofthe chipset next to the CPU can be damaged due to board flexure causedby shock & vibration of the CPU heat sink.

[0005] Stiffening bars have been used to provide mechanical stiffeningfor system substrates to resist sag and flexing. These are essentiallysmall I-beams that are wave soldered to the frontside of the systemsubstrate through holes at regular intervals. The stiffening bars areplaced at strategic locations and orientations on the system substrateto reduce deflection in selected directions. Component placement on thesystem substrate, however, must accommodate placement of the stiffeningbars, putting constraints on the use of available standardized systemsubstrate designs. In addition, the mounting holes required by thestiffening bars are relatively large. In the typical desktop system,stiffening bars are not feasible considering the component densityrequirements of the motherboard. They are used mainly for workstationand server systems that are less size constricted.

[0006] Stiffening plates and load transfer plates are also available.Stiffening plates are plastic or metal plates that attach to thebackside of the system substrate by rivets. The rivets requirerelatively large holes that are difficult to accommodate in smallersystems. Load transfer plates are used to attach the backside of thesystem substrate to the chassis for additional support. Both stiffeningplates and load transfer plates require access to the backside of thesystem substrate and a backside assembly step during manufacturing.Backside assembly is not a process that is normally used in commondesktop motherboard manufacturing, for example, which leads to increasedassembly cost and complexity.

[0007] Improved structural reinforcements are required to mitigatedamage to the interconnects of SMT component due to system substrateflexing. The structural reinforcements must accommodate locations on thesystem substrate that have high trace densities, for example, theCPU/chipset core of a motherboard. Further, the structuralreinforcements must not displace the layout of the components or traces.Further, the structural reinforcements must accommodate the heatdissipation devices used, for example, with core area components.

BRIEF DESCRIPTION OF DRAWINGS

[0008]FIG. 1 is a perspective view of an embodiment of a stiffeningmember in accordance with the present invention;

[0009]FIGS. 2A and 2B are side views of the system substrate and a SMTcomponent with the addition of the stiffening member in accordance withan embodiment of the method of the present invention;

[0010]FIGS. 3A and 3B illustrate the effectiveness of the stiffeningmember to prevent flexing of the system substrate spanned by thestiffening member;

[0011]FIG. 4 is a side view of a system substrate and a SMT componentwith the addition of a stiffening member in accordance with anembodiment of the present invention;

[0012]FIGS. 5 and 6 are perspective and side views, respectively, ofanother embodiment of a stiffening member in accordance with the presentinvention;

[0013]FIG. 7 is a side view of another embodiment of a stiffening memberin accordance with the present invention; and

[0014]FIG. 8 is a side view of another embodiment of a stiffening memberin accordance with the present invention.

DESCRIPTION

[0015] In the following detailed description, reference is made to theaccompanying drawings which form a part hereof wherein like numeralsdesignate like parts throughout, and in which is shown by way ofillustration specific embodiments in which the invention may bepracticed. It is to be understood that other embodiments may be utilizedand structural or logical changes may be made without departing from thescope of the present invention. Therefore, the following detaileddescription is not to be taken in a limiting sense, and the scope of thepresent invention is defined by the appended claims and theirequivalents.

[0016] Embodiments of stiffening members in accordance with the presentinvention provide a mechanical support that is wave soldered to thefrontside of the system substrate simultaneously with other wavesoldered components. The stiffening members do not require backsideaccess or synchronization with chassis design. The stiffening membersconsume very little system substrate space while retaining a platformfor heat dissipation. It can be a platform for chipset thermal solutionor remain a stand-alone mechanical solution.

[0017]FIG. 1 is a perspective view of an embodiment of a stiffeningmember 10 in accordance with the present invention. FIG. 2A is across-sectional view of the stiffening member of FIG. 1. The stiffeningmember 10 comprises a flat plate 12 in the form of a reduced squarehaving an extension 15 at each corner from which a mounting pin 14 iscoupled. The four mounting pins 14 are predetermined to provide theplate 12 with sufficient support to resist expected loading conditionswhen wave soldered in plated through holes 23 on a system substrate 20.The mounting pins 14 are adapted to be inserted into plated throughholes 23 on the system substrate 20. The length of the mounting pins 14are predetermined to account for the height of the SMT component 24 uponwhich it is attached, the thickness of the system substrate 20, and thedesired amount of mounting pin 14 protrusion from the backside 22 of thesystem substrate 20.

[0018]FIGS. 2A and 2B are side views of the system substrate 20 and aSMT component 24 with the addition of the stiffening member 10 inaccordance with an embodiment of the method of the present invention. Asystem substrate 20 is provided having an attached SMT component 24 anda plurality of plated through holes 23 about the periphery of the SMTcomponent 24. A stiffening member 10 is provided having a stiffeningplate 12 from which a plurality of mounting pins 14 project. A topmostportion 27 of the SMT component 24 is provided with a layer of thermalinterface material 28. The mounting pins 14 of the stiffening member 10are inserted into the plated through holes 23 with the plate 12 inabutment with the thermal interface material 28. The assembly isprocessed in a wave solder pre-heat process which softens or melts thethermal interface material 28 to provide intimate thermal contactbetween the SMT component 24 and the plate 12. A wave soldering processis used to solder the mounting pins 14 to the plated through holes 23,to produce solder bonds 29, fixing the stiffening member 10 to thesystem substrate 20 with the plate 12 in thermal contact with the SMTcomponent 24. Wave soldering of the stiffening member 10 is performed inthe same wave soldering operation as for the other wave solderedcomponents.

[0019]FIGS. 3A and 3B illustrate the effectiveness of the stiffeningmember 10 to prevent flexing of the system substrate 20 spanned by thestiffening member 10. FIG. 3A is a side view of a system substrate 20and a SMT component 24 interconnected with BGA interconnects 26. FIG. 3Bis a side view of a system substrate 20 and a SMT component 24 subjectedto a load sufficient to cause flexing of the system substrate 20 underthe SMT component 24. Under loading conditions, the SMT interconnects 26between the system substrate 20 and the carrier substrate 25 of the SMTcomponent 24 is subjected to stresses which can lead to SMT interconnect26 failure.

[0020]FIG. 4 is a side view of a system substrate 20 and a SMT component24 with the addition of a stiffening member 10 in accordance with anembodiment of the present invention. The stiffening member 10 supportsthe system substrate 20 to prevent bending and flexing of the spannedportion 13 under the plate 12. The stiffening member 10 prevents thesystem substrate 20 from deforming in a direction perpendicular to thesystem substrate 20. This is the direction of shock that causes mostdamage to the SMT interconnects 26. FIGS. 3A and 3B and 4 are simplifiedrepresentations; actual performance will depend on other considerations,such as the presence of other components and system substrate 20 tochassis mounting points, among others.

[0021] In other embodiments in accordance with the present invention,the plate 12 comprises other configurations, such as, but not limitedto, a rectangle and a square. The choice of configuration of the plate12 is predetermined to provide sufficient support to the systemsubstrate 20, as well as other considerations, such as, but not limitedto, to provide sufficient support to accommodate an attached heatdissipation device.

[0022] The plate 12 in FIG. 1 has a continuous flat surface. FIGS. 5 and6 are perspective and side views, respectively, of another embodiment ofa stiffening member 11 in accordance with the present invention. Theplate 16 has an aperture 17 to serve as a frame through which a SMTcomponent 24 may protrude. In one embodiment in accordance with thepresent invention, the aperture 17 is of a predetermined size andconfiguration to accommodate the protruding die 27 of a SMT component24, wherein the plate 16 rests on the top surface 21 of the carriersubstrate 25 upon which the die 27 is coupled, without contacting thedie 27.

[0023]FIG. 7 is a side view of another embodiment of a stiffening member30 in accordance with the present invention. The plate 31 has a centralrecess 32 to accommodate the height of the die 27 such that the plate 12is placed in contact with both the die 27 and the carrier substrate 25.The plate 12 is adapted to be in direct contact with the top surface 21of the carrier substrate 25, as well as adapted to accommodate thermalinterconnect material 28 between the plate 31 and the die 27.

[0024] Referring again to FIG. 1, the arrangement of the mounting pins14 about the plate 12 is predetermined, in part, by consideration of thesize of SMT component 24 upon which the stiffening member 10 is mountedand the location of available plated through-holes on the systemsubstrate 20. The number of mounting pins 14 is predeterminedconsidering the amount of support each mounting pin 14 is required toprovide the system substrate 20 and the degree of loading the plate 12is expected to resist.

[0025]FIG. 8 is a side view of another embodiment of a stiffening member34 in accordance with the present invention. The plate 35 furthercomprises heat-dissipation structures 36, such as, but not limited to,fins and pins, which project from the side 37 opposite to the mountingpins 14. The stiffening member 10 provides support for the systemsubstrate 20 as well as provides heat dissipation for the SMT component24 upon which it is coupled.

[0026] In other embodiments of the stiffening member in accordance withthe present invention (not shown), the stiffening member furthercomprises mounting features to accommodate the attachment of a heatdissipation device, such as, but not limited to, a heatsink and a fan.Embodiments of mounting features include, but are not limited to,retention clips and fastener apertures.

[0027] In other embodiments in accordance with the present invention(not shown), heat sinks are provided with wave solderable mounting pinsadapted to be inserted into plated through holes of a system substrate.A heatsink manufactured with an edge flange is fitted with mountingpins. The mounting pins are coupled to the flange using suitablemethods, such as, but not limited to, punch or drill and press-fitinsertion. In another embodiment, the mounting pins are integral to thecasting of the heatsink.

[0028] Finite element analysis (FEA) was used to evaluate theeffectiveness of the embodiment of the stiffening member 10 of FIG. 1subjected to a board-level shock condition consistent with standardtesting procedures. The BGA SMT interconnects 26 of a SMT component 24on an un-reinforced system substrate 20 experienced a maximum tensileload of 1.62 lb at the point of maximum heatsink acceleration. The BGASMT interconnects 26 of a SMT component 24 supported by thewave-soldered stiffening member 10 experienced a maximum tensile load of0.90 lb for the same shock condition. The tensile stress on the solderinterconnections of the ball grid array of the SMT component 24 wasreduced as much as 45% in a shock event compared to an un-reinforcedsystem substrate 20. Performance of the stiffening member 10 can beimproved and optimized with careful consideration to the orientation andplacement of the mounting pins 14.

[0029] Although specific embodiments have been illustrated and describedherein for purposes of description of the preferred embodiment, it willbe appreciated by those of ordinary skill in the art that a wide varietyof alternate and/or equivalent implementations calculated to achieve thesame purposes may be substituted for the specific embodiment shown anddescribed without departing from the scope of the present invention.Those with skill in the art will readily appreciate that the presentinvention may be implemented in a very wide variety of embodiments. Thisapplication is intended to cover any adaptations or variations of theembodiments discussed herein. Therefore, it is manifestly intended thatthis invention be limited only by the claims and the equivalentsthereof.

What is claimed is:
 1. A stiffening member for electronic substratehaving plated through holes, comprising: a flat plate; and a pluralityof mounting pins coupled to the flat plate about the periphery of theflat plate, the mounting pins adapted to be inserted into plated throughholes of the electronic substrate and wave soldered thereto.
 2. Thestiffening member of claim 1, wherein the plurality of mounting pins arespaced apart a predetermined distance, the flat plate adapted to spanacross an electronic component coupled to the electronic substrate. 3.The stiffening member of claim 2, the flat plate adapted to come intothermal contact with an electronic component coupled to the electronicsubstrate.
 4. The stiffening member of claim 1, wherein the flat platefurther comprises an aperture, the aperture adapted to accommodate aprotrusion on an electronic component coupled to the electronicsubstrate, and wherein the flat plate is adapted to contact the carriersubstrate of the electronic component.
 5. The stiffening member of claim1, wherein the flat plate further comprises a recess portion, the recessportion adapted to accommodate and come into thermal contact with aprotrusion on an electronic component coupled to the electronicsubstrate, and wherein the flat plate is adapted to contact the carriersubstrate of the electronic component.
 6. The stiffening member of claim1, wherein the flat plate further comprises a plurality of heatdissipation features.
 7. The stiffening member of claim 6, wherein theplurality of heat dissipation features are selected from the groupconsisting of fins and pins.
 8. The stiffening member of claim 1,wherein the flat plate further comprises one or more features adapted tocouple with a heat dissipation device.
 9. A method for reinforcing anelectronic substrate having plated through holes, comprising: providinga stiffener member, comprising: a flat plate; and a plurality ofmounting pins coupled to the flat plate about the periphery of the flatplate; inserting the mounting pins into cooperating plated throughholes, the flat plate spanning one or more electronic components betweenthe flat plate and the electronic substrate; and coupling the mountingpins to the electronic substrate from the backside of the electronicsubstrate.
 10. The method of claim 9, further comprising placing theflat plate into thermal contact with the electronic component prior tocoupling the mounting pins to the electronic substrate.
 11. The methodof claim 9, wherein coupling the mounting pins to the electronicsubstrate from the backside of the electronic substrate comprises wavesoldering the mounting pins to the electronic substrate from thebackside of the electronic substrate.
 12. The method of claim 9, whereinproviding a stiffener member, comprising a flat plate comprises:providing a stiffener member, comprising a flat plate, the flat platehaving an aperture adapted to accommodate a protrusion on an electroniccomponent coupled to the electronic substrate, and the method furthercomprising placing the flat plate into engagement with the carriersubstrate of the electronic component.
 13. The method of claim 9,wherein providing a stiffener member comprising a flat plate, comprises:providing a stiffener member comprising a flat plate, the flat platehaving a recess adapted to accommodate a protrusion on an electroniccomponent coupled to the electronic substrate, and the method furthercomprising placing the bottom surface of the recess into thermal contactwith a protrusion on an electronic component coupled to the electronicsubstrate, and placing the flat plate into engagement with the carriersubstrate of the electronic component.
 14. The method of claim 9,wherein providing a stiffener member comprising a flat plate comprises:providing a stiffener member comprising a flat plate, the flat platehaving a plurality of heat dissipation features.
 15. The method of claim9, wherein providing a stiffener member comprising a flat platecomprises: providing a stiffener member comprising a flat plate, theflat plate having a plurality of heat dissipation features selected fromthe group consisting of fins and pins features.
 16. The method of claim9, wherein providing a stiffener member comprising a flat platecomprises: providing a stiffener member comprising a flat plate, theflat plate having one or more features adapted to couple with a heatdissipation device; the method further comprising coupling a heatdissipation device onto the flat plate.
 17. A computing system,comprising: a system substrate having plated through holes; and astiffening member, comprising: a flat plate; and a plurality of mountingpins coupled to the flat plate about the periphery of the flat plate,the mounting pins adapted to be inserted into plated through holes ofthe system substrate and wave soldered thereto.
 18. The computing systemof claim 17, the system substrate further comprising one or moreelectronic components, the plurality of mounting pins spaced apart apredetermined distance, the flat plate adapted to span across at leastone of the electronic components coupled to the system substrate. 19.The computing system of claim 19, the flat plate adapted to come intothermal contact with the electronic component spanned by the flat plate.20. The computing system of claim 19, wherein the flat plate furthercomprises an aperture, the aperture adapted to accommodate a protrusionon an electronic component spanned by the flat plate, and wherein theflat plate is adapted to contact the carrier substrate of the electroniccomponent spanned by the flat plate.
 21. The computing system of claim19, wherein the flat plate further comprises a recess portion, therecess portion adapted to accommodate and come into thermal contact witha protrusion on an electronic component spanned by the flat platesubstrate, and wherein the flat plate is adapted to contact the carriersubstrate of the electronic component spanned by the flat plate.
 22. Thecomputing system of claim 17, wherein the flat plate further comprises aplurality of heat dissipation features.
 23. The computing system ofclaim 22, wherein the plurality of heat dissipation features areselected from the group consisting of fins and pins.
 24. The computingsystem of claim 17, wherein the flat plate further comprises one or morefeatures adapted to couple with a heat dissipation device.